In the case of wet steam, both steam-borne, or primary, moisture and moisture deposited on the internal metal surfaces of the steam path, i.e., secondary moisture, cause efficiency losses and the potential for erosion.
The water accumulation on diaphragm surfaces is a complex process which is different for different surfaces. In the context of a steam turbine, a “stage” is comprised of and defined as two rows of airfoils; one stationary and the other rotating with the rotating rows of airfoils disposed downstream of the stationary row of airfoils. For diaphragm outer side walls and airfoils (nozzles) the main driver for water accumulation is centrifugal forces which push water droplets from previous rotating buckets outward. As a result of a large incidence angle between moisture droplets and nozzle leading edges, most of the water deposits (impacts) on the forward portion of the nozzle suction side (convex surface) nearest to the leading edge. As for nozzle pressure side surface, the water accumulation takes place all along the channel (FIG. 3) due to centrifugal forces acting on coarse water and the nearer to the trailing edge, the more water is accumulated on this surface. The accumulated water on the outer side wall and nozzles flows downstream toward the rotating buckets, thus increasing the risk of erosion.
The path of deposited moisture in a steam turbine stage may be tracked as follows. The moisture starts out as either primary moisture or secondary moisture that is carried over from the previous stage or stages of the turbine. With reference to FIGS. 1 and 3, the moisture, shown generally at 10 in FIG. 3, is deposited on the pressure or concave side 12 of the stationary airfoil 14. The moisture is driven by the steam to the stationary airfoil trailing edge 16. The moisture is torn off from there in the form of clusters of water which move in the same direction as but slower than the steam, in the wake behind the stationary airfoils 14. The moisture is then atomized as the relative velocity between it and the surrounding steam reaches a certain threshold. At this point, the moisture is significantly increased in its rate of acceleration while still moving slower than the surrounding steam. This moisture, shown generally at 18 in FIG. 1, impacts the rotating airfoil leading edge 22. In this Figure, W is the bucket rotational speed and VWB is the water velocity relative to the bucket.
Referring to FIG. 2, conventionally a number of radial grooves 24 (typically 3) may be located on a rotating airfoil 20 suction or convex side 26 close to the airfoil leading edge 22 for removing the moisture 18 impacted therein. The disadvantage of grooves 24 is that they only remove moisture that has already caused significant efficiency losses. Indeed, efficiency losses of various kinds are realized when the moisture 10 is first deposited on the stationary airfoil pressure side 12 up to and including moisture 18 interception by the rotating leading edge 22.
In some last stages of steam turbines, due to high speed and high local wetness values, erosion in the tip region is a common occurrence unless protective measures are taken. In general the above described mechanism of moisture accumulation on blade surfaces is well accepted and this concept is the baseline for many moisture removal designs in steam turbines' last stages. Manufacturers generally harden the bucket leading edges near this region or shield them with satellite strips. Other protective measures include removal of water through water drainage arrangements in the nozzle outer side walls (end walls) or through suction slots made in hollow stator blades (airfoils or nozzles). This moisture is then collected in circumferential cavities between the diaphragm and the casing and drains to the condenser.
Previous concepts of moisture removal through the blades are based on extraction of moisture film from blade surfaces through slots due to the pressure difference (drop) between the steam path and the hollow blade inner space. Collected moisture in the diaphragm outer/inner rings is then drained ultimately to the condenser. Thus, this prior art consists of providing a hollow diaphragm structure with extraction slot/bores located on the suction side of the airfoil and connected to outer/inner ring chambers that drain to the condenser.
Existing hollow blade extraction slot/bore designs provide moisture removal and thus perform a positive function. However, this moisture evacuation process can work effectively only under sufficient pressure drop, which simultaneously evacuates a significant amount of steam from the steam path to the condenser, thus reducing steam turbine efficiency. Typical values of stream leakage are 0.5-0.8% of main steam. This leakage proportionally reduces steam turbine efficiency.
Another concept to reduce the risk of erosion is based on the fact that water droplets of smaller size cause less erosion than bigger ones. According to this concept, steam is ejected from hollow blade slots to push away the moisture film from the blade surface and at the same time break-up/fragment big water droplets into smaller ones. Conventionally, according to this concept, the steam ejecting slot is positioned as close to the trailing edge as possible.